Installation for the descaling of metals

ABSTRACT

Metals having metal oxide scales are descaled with molten sodium metal. The descaling can be preceded and followed by treatment of the metal with molten alkali metal hydroxide.

United States Patent Rudolf Kuhn Koppern, Taunus, Germany Feb. 16,1968

Jan. 5, 1971 Deutsche Gold-und Silber-Scheideanstalt vormals RoesslerFruitful-tam Main, Germany [72] inventor [21] Appl. No. I 22] Filed [45]Patented [73 Assignee [32] Priority Feb. 16, 1967, July 13, 1967 [331Germany [31 N05. D52,292 and 1153.583

[54] 1 INSTALLATION FOR THE DESCALING OF METALS 6 Claims; 2 DrawingFigs.

52 us. Cl. 134/64, 134/2, 134/15, 134/114 [51] Int. Cl B08b 3/08 [50]Field ofSearclI 134/2,3, 15, 64,114,122

[56] References Cited UNITED STATES PATENTS 876,182 1/1908 Herrick134/64X 1,595,237 8/1926 Minton 134/15X 2,134,457 10/1938 Tainton m.134/15X 2,390,007 1 1/ 1945 Sherman 134/64X 2,601,863 7/1952 Murphy134/15 2,664,901 l/ 1954 Gehr et a1. 134/64X 3,067,758 12/ 1962 Hersh134/64X 3,129,712 4/1964 Thomas 134/64X 3,293,159 12/1966 Mekjean et a1.134/2X 3,358,980 12/1967 Taylor 134/122X 3,424,614 1/1969 Lichte 134/15Primary Examiner-Morris O. Wolk Assistant Examiner-Joseph T. ZatargaAttorney-Stephens, Huettig and O'Connell ABSTRACT: Metals having metaloxide scales are descaled with molten sodium metal. The descaling can bepreceded and followed by treatment of the metal with molten alkali metalhydroxide.

PATENTEU JAN 5 IBYI INVENTOR Rudblf Kuhn @ZM M azawzz ATTORNEYSINSTALLATION FOR THE DESCALING OF METALS BACKGROUND OF THE INVENTION 1.Field of the Invention The invention relates to the removal of metaloxide scales from metals.

2. Description of the Prior Art It is known that hot or cold formedmetal, such as rolled steel bands or drawn wire, must be'annealed, inorder to obscales are removed by having the metalobject pickled in acid.

The treatment of steel objects in an acid pickling process for theremoval of scales therefrom, 'however, gives rise to a number ofdisadvantages. Not-only does the waste water from the process createdisposal and handling problems, as are common to all picklinginstallations, but, due to the fact that the through-put time forthest'eel in the acid pickling bath must be relatively long, when steelband or wire must be passed through the bath at a high rate of speed,then extraordinarily large pickling installations have to be provided inorder to accommodate the needs of the process. Furthermore, andparticularly in those cases where highly alloyed'steel is beingprocessed,the pickling acid not only attacks the scales but the basematerial as well, and thus leads to expensive losses of metal. Inaddition, the acid pickling process also leads to the absorption ofhydrogen by the metal being treated which causes undesirableembrittlement of the metal.

Because of such disadvantages in the use of aqueous pickling baths, saltbath processes have been developed which are significantly short andhave more limited process times than is required with the'acid treatmentprocesses. 'Such salt bath processes include those involving the use ofmolten alkaline salts, in which the scales are more or less given apreremoval treatment with the aid of direct electrical current in orderto assure the quick removal of the scales in a subsequent heat treatmentin the molten salt, as .well as processes in which a to be treated forthe removal of scales therefrom.

All these prior art processes, however, including those which involvethe use of reducing agents, have the disadvantage that they do notremove all-the metal oxide scales from the surfaces of the treated metalbands without the use of 'a subsequent acid pickling treatment, andwhere a reducing agent is used.an acid pickling pretreatment is alsorequired.

- SUMMARY OF THE INVENTION An object of the present invention is toprovide an installation in which to conduct the novel process of thisinvention.

A further object o f-the present invention is to provide an in- I'stallation in which to conduct thenovel process of this invention.

The essence of the process of the present invention involves treatingthe metal to bedescaled with molten sodium metal, which treatment ispreferably both preceded and followed by treating the metal with moltenalkali metal hydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of aninstallation in which the process disclosed maybe conducted.

FIG. 2 is an end view of the installation of FIG. 1.

DESCRIPTION OF THE PREFERREDEMBODIMENTS It has now been unexpectedlyshown, that by treatment thereof with molten sodium metal, and inparticular at temperatures between about 230 and 700 C. and preferablyat temperatures above 300 C., all types of steel, the unalloyed as wellas the highly alloyed types, can be readily descaled. The reduction ofthe metal oxide scales is so complete with such treatment thatsubsequent acid pickling treatments are usually not necessary. Theprocess has important advantages, therefore, in that material losses dueto overpickling are avoided, even if for some reason the metal objectbeing treated must remain stationary in the molten sodium, and in thathydrogen absorption by the metal, which is associated with acid picklingtreatments, does not occur. In addition, due to the direct descaling ofthe metal by means of the sodium metal, the reducing time is confined toonly a few seconds, so that through-put speeds can be achieved for thedescaling process which have not been possible to date The molten sodiummetal treatment is preferably conducted at 230 to 700 C in about 60 to 5seconds.

The molten sodium treating process can be further improved if the metalto be treated is pretreated or dipped in molten alkali metal hydroxidefor a short time before the molten metal treatment. The alkali metalhydroxide used is preferably sodium hydroxide. Molten mixtures of alkalimetal hydroxides and alkali chlorides can also be used. The pretreatmentwith alkali metal hydroxide is preferably conducted at 230 to 700 C. for5 to 60 seconds. The wetting and resulting descaling of the surface ofthe metal being treated is so free of objectionable features that thedescaling process is completed within a very short time without anytrouble.

The alkali metal hydroxide bath which-is used in conjunction with theactual pickling process, which latter process is conducted in the moltensodium metal, can be used as means for sealing the sodium metalprocessing chamber from the atmosphere. The alkali metal hydroxide meltwhich is used for pretreating the object to be descaled can also be usedto pre heat such objects. An alkali metal hydroxide bath can also beused as a post-descaling treatment in order to act as a scavenger andremove from the metal object being processed not only pieces of sodiummetal that may adhere to the metal object as it leaves the sodium metalbath; but also sodium oxide reaction products that may form during thedescaling step. The pieces of sodium metal and sodium oxide reactionproducts that are taken up by the alkali metal hydroxide bath can bereadily converted into sodium hydroxide upon contact with moisture inthe atmosphere.

The molten sodium metal bath is preferably blanketed with an inert gaswhich will not react with the sodium metal. The purpose of this inertgas is primarily to seal off the molten sodium metal bath from gases orother components of the atmosphere which would either react with thesodium metal or with the metal being processed under the prevailingprocess conditions. Examples of the undesired gases or other compoundsare oxygen, hydrogen, water. Nitrogen is the preferred inert gas to beused as the blanketing medium.

A metal processing installation in which the process can be carried outcan be constructed in such a way that the molten sodium metal is allowedto float on top of the molten alkali metal hydroxide bath. The sodiummetal bath can be isolated from the atmosphere by means of a tower typestructure erected over the sodium metal bath. Elongated metal objects,such as, wire and bands, to be treated in the sodium metal bath can befed continuously through the bath and the tower type structuresurrounding the bath by means of feeder devices,

' such as guide rollers.

shaped container has a wide open top and a narrow open bottom whereby aninterface between molten sodium metal and molten alkali metal hydroxidesets in inside the narrow open end of the funnel shaped container.

An example of such an installation is shown in the drawings. FIG. 1shows a side view of the installation and FIG. 2 shows an end view ofthe same installation. The installation consists essentially of afurnace which has a wall 1 and a heating chamber la, an open toppedtrough or vat 2 for the molten alkali metal hydroxide 2a, an open toppedcontainer 3 for the molten sodium metal bath 3a, and airtight housing 4and feeder or guide rollers 5.

Walls 4a of housing 4 extend on all sides down into bath 2a and housing4 provides in combination with the molten alkali metal hydroxide 2a, anair tight area 7. Area 7 is thus bounded on its top and sides by housing4 and at its base by the top of bath 2a. Area 7 completely isolates bath30 from contact with the atmosphere, and during the operation of theinstallation area 7 is charged with an inert gas. The portions of thetop of bath 2a which are outside walls 4a of housing 4 are in contactwith the atmosphere.

An elongated metal object which is to be treated using the process ofthe present invention is fed into and through the installation, in thedirection of the arrow, by means of guide rollers 5. As shown by thedrawings, the metal object passes first through the NaOH bath, thenthrough the Na bath and again through the NaOH bath before it leaves theinstallation. The furnace maintains both the NaOH bath and the Na bathin a molten condition. The NaOH bath is heated directly through itscontainer wall 2 by means of heat generated in heating chamber In. Thesodium is in turn heated by the molten NaOH.

The sodium bath 3a is housed in funnel shaped container 3. Container 3may be suspended from above in the NaOl-l bath or it may be supported bya base which is itself resting on the bottom of trough 2. The walls ofthe lower half of container 3 slope inwardly to provide a narrow, neck 6which is open at the bottom thereof. Neck 6 extends into the NaOH bath2a in such a way as to leave a distance, on all sides of it, between itand the walls and base of trough 2. As a result of this arrangementthere is no horizontal wall separating the molten sodium in the bottomof neck 6 from the molten NaOH in trough 2 so that the molten sodium (y0.95 kg/dm) floats on the molten sodium hydroxide ('y 1.8 kg/dm).Container 3 is thrust so far into the molten NaOh that the interfacebetween the molten sodium and the molten NaOl-l is inside neck 6. Thus,since neck 6 has a definite length, care must be taken during theoperation of the installation to provide that fluctuations in theamounts of sodium and sodium hydroxide in their respective containers donot cause the interface between the sodium and the sodium hydroxide tobe located outside of neck 6.

By reason of the arrangement of the installation as shown in thedrawings various advantages are obtained in conducting the process ofthe present invention in such an installation. When using the disclosedinstallation products such as the M1 0 reduced metals, and impuritieswhich are produced or formed during the descaling step in the moltensodium metal eventually end up in the molten sodium hydroxide. Thisarises due to the fact that such materials are all heavier than themolten sodium. They sink or slide, therefore, to the bottom of container3 and through the open bottom of neck 6 into the molten sodiumhydroxide. Such materials either then further fall to the bottom oftrough 2 or they are taken up by the molten sodium hydroxide. The sodiumoxide materials will dissolve in the molten sodium hydroxide. Thedissolved N320 will not be an impurity in the molten sodium hydroxidesince the Na O will react with moisture in the air that is absorbed intothe molten sodium hydroxide and will form sodium hydroxide. i.e.. Na O-jH O- 2 NaOH. The moisture in the air can enter the sodium hydroxidewhere the molten sodium hydroxide is exposed to the atmosphere. i.e..that portion of the molten sodium hydroxide bath which is outside walls411. The usual impurities which are present or form during the descalingstep. notably reduced metal;

two materials and thereby prevents a large scale dissolution of thesodium in the alkali metal hydroxidel' Since the lower portion ofcontainer 3 is completely immersed in the molten sodium hydroxide, themolten sodium hydroxide can be used as an air tight sealing means, incombination with airtight housing 4, in order to keep moisture and othermaterials in the air from reaching and reacting with the molten sodium.Housing 4 is so constructed that continuous metal wire or bands can beprocessed therein under an inert atmosphere from the time the wire orband enters the molten sodium hydroxide for the first time until itleaves the molten sodium hydroxide at the end of the process. in thisway slits are not needed in walls 4a in order to provide ingress andegress means for the wire and bands. Particles of sodium metal that mayadhere to the band after the band leaves the molten sodium bath arereadily taken up in the molten sodium hydroxide the second time the bandpasses through bath 2a, and thus before the band passes out into theoutside atmosphere.

As can be seen from FIG. 2, housing 4 is constructed wide enough so thatthere is sufficient open space on the surface of bath 20 between walls4a of housing 4 and walls 3b of container 3 as to allow for the removalof impurities in the form of sludge from under container 3 in bath 2athrough such open surface space.

Instead of using a single common bath housed in a single trough fortreating the continuous metal objects in two separate molten alkalimetal hydroxide bath treatments, two separate alkali metal hydroxidebaths placed in separate troughs can also be used. In such instance thesodium melt could also be housed in a separate trough shaped containerof its own and the three troughs could still all be housed under asingle air tight housing.

Those elements of construction which are to be used in the building ofthe installation and which are to come in contact with molten sodiumduring the operation of the installation, such as the inside ofcontainer 3 or the guide rollers which are partly immersed in bath 3a,must be made of a material which will not be attacked by the moltensodium. Such elements of construction should, therefore, be constructedof a sodium resistant material such as nickel, or be plated with suchsodium resistant materials.

The elements of construction which are to come in contact with themolten alkali metal hydroxide should be such as are capable ofwithstanding attack from such hydroxides.

The sealing of the guide roller shafts 8 which are to pass throughhousing 4 can be done with stuffing' Boxes in order to maintain the airtight integrity of housing 4. The heat generated in the heating area 10of the furnace can be generated electrically, as by using resistanceelements, or by burning gas or oil. Heat generating electrodes can alsobe in serted directly into the baths.

The following examples are merely illustrations of th process and arenot intended as a limitation upon the scope thereof.

Example 1 A test piece of ST-high grade steel was heat rolled andannealed in the atmosphere and oxide scales appeared on the" surface ofthe test piece. The surface of the testp'iece was then treated for 5 to10 seconds at 400 C. in molten sodium hydroxide (=l00 percent pure) andthen for 5 to l0 seconds at 450 C. in molten sodium metal percent-pure).The stirface of the thus treated'm'etal piece wasclear ofoxide'sca'les:-'

When test pieces of the same type of heat rolled and annealed ST steelwere descaled in the Customary pickling bath processes using a HCL or H50. pickling-bath heated to 40 to 80 C. the treatments required apickling bath time of at least two minutes in order to achieve thesamedescaling results as were obtained with the process of the presentinvention. 0.4 percent C, 1 percent Mn. t

Example 2 Test pieces of a highly alloy chromesteel identified asworkpiece numbers 4006 to 4021 were heat rolled and annealed in theatmosphere whereupon oxide scales appeared on the surface of the testpieces. The surface of some of the test pieces were then treated for 5to 10 sec. at 500 C. in molten sodium hydroxide 100 percent pure) andthen for 5 to l seconds at 400 to 500 C(inmolten sodiummetal 100 percentpure). The surface of the thus treated metal pieces were clear of oxidescales.

When some other of the test pieces were descaled in a pickling treatmentusing a mixture of HNO; and HF as the pickling bath at 40 to 60 C. thetreatment required a pickling bath time of more than minutes in order toprovide the same descaling effect as was provided by the process of thepresent invention. 18 percent Cr.

A test piece of highly alloyed chrome-nickel steel identified as workpiece number 4300. was hot rolled and annealed in the atmospherewhereupon oxide scales appeared on the surface of the test piece. Thesurface of the test piece was then treated for 5 to l0 sec. at 400 C. inmolten sodium hydroxide (100 percent pure) and then for 5 to seconds at500 C. in molten sodium metal.(l00 percent pure). The layer of oxidescales was thus removed from the surface of the test piece.

When a test piece of the same type of'steel as workpiece 4300 which hadbeen hot rolled and annealed as the piece used above was .descaled in apickling treatment using a mixture of HNO and HF as the pickling bath at40 to 60 C. the treatmentrequired a pickling bath time of 5 minutes inorder to provide the same descaling effect as was provided by theprocess of the present invention. l8 percent Cr 8 percent Ni The moltensodium bath should contain sodium which is at least 100 percent pure.The alkali metal hydroxide bath should contain alkali metal hydroxidewhich is at least 100 percent pure.

The metals which may be descaled in accordance with the presentinvention include alloyed and nonalloyed iron and steel as well ascopper, alloys of zirkonium and titanium, iron free alloys basing on Ni,Co and Cr. g

The metals being processed in the process of the present invention mayhave any shape or configuration. The process is particularly suitable tothe treatment of objects which may be fed into themolten sodium metalbath in a continuous fashion, such as wire, cable, strips, bands,sheets, and similar elongated objects.

l. A metal object treating apparatus comprising, in combination, a firstcontainer, a bath of molten alkali metal hydroxide in said containersuitable for pretreatment of the surface of said object, a secondcontainer open at the. top, a bath of molten sodium metal in said secondcontainer suitable for the treatment of the surface of said object aftersaid baths in a molten state during said pretreatment and treatment,feeder means for feeding a metal object in and out of said baths,airtight housing means. and an inert atmosphere in said housing meansover at least said bath of molten sodium metal and in contact therewithat said open top.

2. An apparatus according to claim 1 wherein the cross-sectional area ofsaid housing means is less than the cross-sectional area of the uppersurface of said first container and the bottom of said housing meansextends into said first container above the bottom thereof whereby whensaid first container contains a molten alkali metal hydroxide bath, saidbath seals said housing means, and the open top of said second containerextends above the top of said first container, said housing meansproviding a space for an inert atmosphere above the area of the open topof said second container and at least the immediately adjacent areaabove said first container.

- tionallarea of said first container, the molten alkali metal hydroxidebath being in contact with the molten sodium metal bath and sealing thebottom of saidsecond container, said molten sodium metal bath floatingon said molten alkali metal hydroxide bath, heating means formaintaining both baths in a molten state, feeder means for feeding ametal object sequentially through saidvfirst bath, then through saidsecond bath and again through said first bath, airtighthousing means, aninert atmosphere within said housing means, said opening of said widetop'of said second container disposed above the level of the moltenalkali metal hydroxide in said first container'and extending into saidinert atmosphere within the cross-sectional area of said airtighthousing means whereby an inert atmosphere is maintained above saidsodium metal.

4. An apparatus as in claim 3 inwhich all surfaces of said secondcontainer which contact sodium metal are made of nickel or are nickelplated.

5. An apparatus according to claim -3 in which said alkali metalhydroxide is sodium hydroxide.

6. An apparatus suitable for treating a metal object in a plurality ofbaths, one of said baths. beinga molten alkali metal tainer beingpositioned within, but out of contact with said pretreatment said secondcontainer having a narrow, open ,7

bottom, said bottom being'positionedwithin, but out of contact with saidfirst container, the opening of said bottom being disposed withinthe'intemal cross-sectional area of said first container, .the moltenalkali metal hydroxide-bath being in second container, the opening ofsaid narrow bottom being disposed within the internal crosssectionalarea of said first container, heating means suitable for maintainingboth of said baths in molten state, feeder means for'feeding a metalobject sequentially through said first container, then throughsaidsecond container and then again. through said first container,airtight housing means, comprising a top and sidewall, said wide top ofsaid second container being disposed within the cross-sectional area ofsaid housing means and below the top of said housing means.

2. An apparatus according to claim 1 wherein the cross-sectional area ofsaid housing means is less than the cross-sectional area of the uppersurface of said first container and the bottom of said housing meansextends into said first container above the bottom thereof whereby whensaid first container contains a molten alkali metal hydroxide bath, saidbath seals said housing means, and the open top of said second containerextends above the top of said first container, said housing meansproviding a space for an inert atmosphere above the area of the open topof said second container and at least the immediately adjacent areaabove said first container.
 3. A metal object treating apparatuscomprising in combination a first container open at the top thereof, amolten alkali metal hydroxide bath in said container suitable for thepretreatment of the surface of said metal object, a second, funnelshaped, container having a wide, open top and a narrow, open bottompositioned within, but out of contact with said first container, amolten sodium metal bath in said second container suitable for thetreatment therein of the surface of said metal object after saidpretreatment, the opening of said narrow bottom being disposed withinthe internal cross-sectional area of said first container, the moltenalkali metal hydroxide bath being in contact with the molten sodiummetal bath and sealing the bottom of said second container, said moltensodium metal bath floating on said molten alkali metal hydroxide bath,heating means for maintaining both baths in a molten state, feeder meansfor feeding a metal object sequentially through said first bath, thenthrough said second bath and again through said first bath, airtighthousing means, an inert atmosphere within said housing means, saidopening of said wide top of said second container disposed above thelevel of the molten alkali metal hydroxide in said first container andextending into said inert atmosphere within the cross-sectional area ofsaid airtight housing means whereby an inert atmosphere is maintainedabove said sodium metal.
 4. An apparatus as in claim 3 in which allsurfaces of said second container which contact sodium metal are made ofnickel or are nickel plated.
 5. An apparatus according to claim 3 inwhich said alkali metal hydroxide is sodium hydroxide.
 6. An apparatussuitable for treating a metal object in a plurality of baths, one ofsaid baths being a molten alkali metal hydroxide bath and a second ofsaid baths being a molten sodium metal bath disposed on top of saidalkali metal hydroxide bath and covered with an inert atmosphere, saidapparatus comprising, in combination, a first lower container open atthe top thereof suitable for holding said molten alkali metal hydroxide,a second, funnel shaped container suitable for holding said moltenalkali metal, said second container having a wide, open top and anarrow, open bottom, said second container being positioned within, butout of contact with said second container, the opening of said narrowbottom being disposed within the internal cross-sectional area of saidfirst container, heating means suitable for maintaining both of saidbaths in molten state, feeder means for feeding a metal objectsequentially through said first container, then through said secondcontainer and then again through said first container, airtight housingmeans, comprising a top and sidewall, said wide top of said secondcontainer being disposed within the cross-sectional area of said housingmeans and below the top of said housing means.